1. Field of the Invention
The present invention relates to a solid polymer electrolyte fuel cell assembly including a plurality of unit cells integrally stacked to each other, wherein each of the unit cells has a unified body formed by holding a solid polymer electrolyte membrane between an anode and a cathode, a fuel cell stack obtained by stacking the solid polymer fuel cell assemblies to each other, and a method of operating the solid polymer electrolyte fuel cell assembly.
2. Description of the Related Art
In general, a solid polymer electrolyte fuel cell (PEFC) includes a unit cell (unit power generation cell) formed by disposing an anode and a cathode, each of which is mainly made from carbon, on both sides of an electrolyte membrane of a polymer ion exchange membrane (cation exchange membrane), to form a unified body (membrane-electrode assembly), and holding the unified body between separators (bipolar plates). The solid polymer electrolyte fuel cell is generally used as a fuel cell stack having a specific number of the unit cells.
In the fuel cell of this type, when a fuel gas, for example, a gas mainly containing hydrogen (hereinafter, referred to as “hydrogen containing gas”) is supplied to the anode, hydrogen in the hydrogen containing gas is ionized on the catalyst electrode and is migrated to the cathode via the electrolyte; and electrons generated by such electrochemical reaction are taken to an external circuit, to be used as electric energy in the form of a direct current. In this case, since an oxidizing gas, for example, a gas mainly containing oxygen or air (hereinafter, referred to as “oxygen containing gas”) is supplied to the cathode, hydrogen ions, electrons and oxygen react with each other to produce water on the cathode.
When a fuel cell stack is used as an on-vehicle power source, a relatively large output is required for the fuel cell stack. To meet such a requirement, a cell structure for making a size of a reaction plane (power generation plane) of a unit cell larger, and a cell structure for stacking a large number of unit cells to each other have been adopted.
The former cell structure, however, has a problem that if the size of each unit cell becomes large, the whole size of the fuel cell stack also becomes large, and such a large-sized fuel cell stack is unsuitable as an on-vehicle power source. Accordingly, the latter cell structure for stacking a large number of relatively compact unit cells to each other has been generally adopted. However, as the number of the stacked unit cells becomes large, the temperature distribution tends to be generated in the stacking direction and also the drainage characteristic of water produced by the electrochemical reaction is degraded, thereby failing to ensure a desired power generation performance.